Recently, substantial attention has been directed to the field of superconductors and to systems and methods for using such products. Substantial attention also has been directed to systems and methods for providing a cold environment (e.g., 77.degree. K or lower) within which superconductor products such as superconducting filter systems may function.
One device that has been widely used to produce a cold environment within which superconductor devices may function is the stirling cycle refrigeration unit or stirling cycle cryocooler. Such units typically comprise a displacer assembly and a compressor assembly, wherein the two assemblies are in fluid communication and are driven by one or more linear or rotary motors. Conventional displacer assemblies generally have a "cold" end and a "hot" end, the hot end being in fluid communication with the compressor assembly. Displacer assemblies generally include a displacer having a regenerator mounted therein for displacing a fluid, such as helium, from one end, i.e., the cold end, of the displacer assembly, to the other end, i.e., the hot end, of the displacer assembly. The piston assembly functions to apply additional pressure to the fluid, when the fluid is located substantially within the hot end of the displacer assembly, and to relieve pressure from the fluid, when the fluid is located substantially within the cold end of the displacer assembly. In this fashion, the cold end of the displacer assembly may be maintained, for example, at 77.degree. K, while the hot end of the displacer assembly is maintained, for example, at 15.degree. K above ambient temperature.
Now, in situations where it may be desirable to drive the drive the compressor and displacer assemblies using a single linear motor, and where it is desired to have the compressor assembly operate at a fairly high oscillation frequency, such as 60 Hz, several issues arise with regard to the design of the motor assembly. For example, it has been found that, when a conventional linear motor is used to drive the compressor and displacer assemblies of a cryocooler, it is extremely difficult to achieve a 60 Hz operating frequency for the compressor and displacer assemblies. One reason for this is that the diameter of the piston of a given compressor and the amount of displacement achieved by the displacer in a given displacer assembly dictate, to a very large degree, what the maximum amount of moving mass within an associated motor may be. If the moving mass is too great, the motor and associated compressor and displacer assemblies will not function properly.
Thus, it is believed that those skilled in the art would find a linear motor capable of operating at, for example, a 60 Hz resonant operating frequency, when coupled to a piston assembly used in a typical cryocooler to be quite useful.